1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus advantageously configured to horizontally mix signal charges read out from photosensitive cells for thereby thinning pixels, and a signal reading method for the same.
2. Description of the Background Art
A digital camera, for example, includes an area or bidimensional image sensor for picking up a desired scene. Recently, a high-resolution area image sensor having as many as several million photosensitive cells for electric transduction, i.e. pixels, has been put on the market for implementing higher image quality. While a camera mode or still picture mode and a movie mode are available with a digital camera, image pickup in the movie mode is periodically effected at preselected time intervals in order to display consecutive frames in the form of movie. The movie mode is sometimes referred to as a monitor mode in the sense that the operator of the camera uses it when determining which scene is viewed into a still picture.
If signal charges captured by and stored in the high-resolution area image sensor in its monitor are directly read out, then the reading operation may not complete within the preselected period of time. In light of this, it is a common practice to limit the number of pixels to be read out to one that can be followed. More specifically, signal charges are read out from a plurality of photosensitive cells arranged in lines extending in the horizontal direction of a picture frame, or the direction of row, and then mixed or added in the vertical direction. The vertical addition of the signal charges increases the amount of signal charges, so that pickup sensitivity is enhanced in dependence upon the amount of signal charge.
To allow the area image sensor to pickup a color image, a color filter is positioned on the array of photosensitive cells of the image sensor for separating incident light into different color components. Generally, as for a digital still camera, a single color filter is assigned to a single image sensor and has color filter segments arranged in one-to-one correspondence to the photosensitive cells. Therefore, in a digital still camera with a single color filter, signal charges derived from the photosensitive cells associated with the color filter segments of the same color are mixed in the vertical direction. Such vertical thinning, however, lowers the vertical resolution of a picture.
In the future, area image sensors having a further increased number of pixels may be required to cope with reading out signal charges in the monitor mode. Then, consideration may be given not only to vertical mixture but also to horizontal mixture. In this respect, Japanese patent laid-open publication No. 2001-86519, for example, teaches a solid-state image pickup apparatus paying attention to a positional relation between lines to be mixed. The image sensor taught in this document generates, e.g. three primary colors R (red), G (green) and B (blue) out of color-mixed signals output from an area image sensor and then executes sophisticated calculations for special image processing with the color mixed-signals later.
Horizontal thinning in the area image sensor mixes pixels in the horizontal direction, so that the resulting pixel signals constitute a pixel space, or image, of new colors. The pixel space is obviously representative of a color space different from the conventional pixel positions and unbalanced in pixel arrangement. Should the conventional signal processing be executed with such an unbalanced pixel arrangement without the special image processing mentioned above, vertical stripes, false colors and other image defects would appear in the resulting picture, degrading image quality.
In the image pickup apparatus disclosed in laid-open publication No. 2001-86519 mentioned earlier, the colors must be mixed in a particular condition determined by a procedure used for reproducing the colors R, G and B from image data derived from the horizontal mixture. The limited condition available for color mixture will be problematic when higher signal reading speed is desired.